Wireless communication systems are widely used to provide various types of communications. For example, voice and/or data are provided by the wireless communication systems. A conventional wireless communication system provides multiple users with one or more shared resources. For example, the wireless communication system can use various multiple access schemes such as code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA).
The institute of electrical and electronics engineers (IEEE) 802.16 standard provides a technique and protocol for supporting broadband wireless access. The standardization had been conducted since 1999 until the IEEE 802.16-2001 was approved in 2001. The IEEE 802.16-2001 is based on a physical layer of a single carrier (SC) called ‘WirelessMAN-SC’. The IEEE 802.16a standard was approved in 2003. In the IEEE 802.16a standard, ‘WirelessMAN-OFDM’ and ‘WirelessMAN-OFDMA’ are further added to the physical layer in addition to the ‘WirelessMAN-SC’. After completion of the IEEE 802.16a standard, the revised IEEE 802.16-2004 standard was approved in 2004. To correct bugs and errors of the IEEE 802.16-2004 standard, the IEEE 802.16-2004/Cor1 was completed in 2005 in a format of ‘corrigendum’.
A system profile based on the conventional IEEE 802.16 standard supports only a time division duplex (TDD) scheme. In the TDD scheme, uplink transmission and downlink transmission are achieved at different times while occupying the same frequency band. Since an uplink channel and a downlink channel have almost reciprocal characteristics, the TDD scheme has an advantage in that frequency selective scheduling is simple.
A frequency division duplex (FDD) and/or half-duplex FDD (H-FDD) scheme is expected to be introduced in the IEEE 802.16 standard. A user equipment (UE) using the FDD scheme can simultaneously perform uplink transmission and downlink transmission on different frequency bands. A UE using the H-FDD scheme cannot simultaneously perform uplink transmission and downlink transmission on different frequency bands. Therefore, when uplink transmission is performed by UEs belonging to one group in an H-FDD system, a base station (BS) performs downlink transmission for UEs belonging to another group. That is, frequencies are divided for uplink transmission and downlink transmission, and times are divided to be used by respective groups.
A midamble is a channel estimation signal transmitted by the BS to obtain a channel state for each antenna in a multiple-input multiple-output (MIMO) system using a plurality of antennas. By receiving the midamble, a UE can estimate the channel state of the BS for each antenna. In a TDD-based frame structure, a first orthogonal frequency division multiplexing (OFDM) symbol in a spatial time code (STC) zone is defined as a position at which the midamble is transmitted. In case of using an FDD or H-FDD-based frame structure, limited radio resources may be wasted when the BS transmits the midamble every time for all groups.
Accordingly, there is a need for a method of transmitting a midamble by using minimum radio resources for transmission of the midamble.